Difference between revisions of "Recording length"

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==Recording time/disk space remaining==
==Recording time/disk space remaining==
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==Increasing disk space for longer recordings==  
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==Recordings over 13.5 hours==
==Recordings over 13.5 hours==

Revision as of 18:34, 19 July 2009

Audacity lets you record for as long and as often as you need, subject to the disk space you have available, and (in 1.2.x), a limit of 13.5 hours at default 44100 Hz sample rate. The disk space needed for recording depends on the quality (sample rate and bit depth) you record at.
Related article(s):

Recording time/disk space remaining

When recording, the audio data is written to your hard disk. For example, if you just press the red Record button without having yet saved a Project, it's written to Audacity's temporary folder. Therefore, the amount of time you can record for is limited by the available disk space on the drive you are writing to. Audacity lets you know the amount of time you can record for based on that remaining space. Older versions of Audacity had a confusing "recording time remaining" message which did not make this clear. However current versions of Audacity which you can always download here  give a clear indication in the Status Bar at the bottom of the screen as to how much time you can record for:

Status Bar showing disk space remaining for recording

The time remaining depends on the current space on the drive, and on the quality (sample rate and bit depth) you are recording at.

Warning icon Beta 1.3.3 and later has an option on the Directories tab of Preferences to cache the recorded data to memory during recording: writing to the temporary folder only occurs after recording is stopped. This can be useful if your temporary folder resides on a slow network drive, but note that currently this option also caches most audio data including project edits, so can cause crashes.

Although there are rarely problems if you record to Audacity's temporary folder and then save the recording as an Audacity Project, it may be preferable if you're intending to save a Project to save an empty Project before you start recording, with the File > Save Project As... command. That way, the recorded data will be saved to a permanent location in the Project's _data folder. When the recording is over, and now and again thereafter, use File > Save Project (not File > Save Project As...) to update the Project.

Increasing disk space for longer recordings

To get more recording time:

  1. Obviously, free up some more disk space. One good way to do that is to ensure you delete your old Audacity Projects (the .aup and .aup.bak files and the _data folders) as soon as you've finished the Project and are sure you won't need to export any more files from it.
  2. Save the Project you're starting (or change the location of Audacity's temporary folder) to a disk with more space on it. The location of Audacity's temporary folder can be changed on the Directories tab of Preferences.
  3. Audacity only uses uncompressed (so lossless) PCM audio data for recording and editing, but the amount of disk space consumed can be reduced by changing the following settings in Preferences:
    1. on the Audio I/O tab, record in mono instead of stereo. This halves the disk space used.
    2. on the Quality tab, reduce the Default Sample Format (bit depth) from 32 bit float to 16 bit, which again halves the disk space used. In speech recordings you may also be able to reduce the Default Sample Rate without significantly affecting the quality. Generally for music, leave the Sample Rate at 44100 Hz, but if 22050 Hz gives acceptable quality, it takes only half the disk space of 44100 Hz.
  4. Note that because unlimited undo is available in an Audacity session, editing a project itself uses disk space. To reduce the space a project is taking, File > Save Project, exit Audacity and relaunch it. This will clear out the space being used by the Undo mechanism. Alternatively, leave the project open, click View > History and discard the Undo levels you do not need.

Examples of disk space usage when recording and editing at different quality settings:
  • 44100 Hz, 32 bit, stereo = 20 MB of space per minute. 44 100 Hz and 32 bit are Audacity's default quality settings
  • 44100 Hz, 16 bit, stereo = 10 MB per minute. CD quality
  • 22050 Hz, 8 bit, mono = 1.25 MB per minute. This would be generally acceptable for speech recordings from lower quality sources

Note that disk space usage above relates solely to the size of audio data held by Audacity, not to the size of audio files exported from it. You need this space to record and edit , but if you don't save an Audacity Project, the temporary data is deleted and the space released as soon as you exit Audacity.

When making a longer recording of a few hours or more, other computer resource issues may be involved apart from disk space. These include the need to have sufficient RAM and swap file space available, and ensuring automated events such as system or program updates or screen savers are not able to disrupt a recording. For details, see Managing Computer Resources and Drivers. Having stable, up-to-date sound device drivers properly matched to your hardware also becomes more important .

Recordings over 13.5 hours

There is a limit to recording time in Audacity 1.2.x which is determined by the maximum number of audio samples it can store. When recording at Audacity's default 44100 Hz sample rate this implies a maximum recording time of about 13 hours, 31 minutes, 35 seconds. The reason is that this length of time (48695 seconds), at a 44100 Hz sample rate (44100 samples per second), gives you 2 147 449500 samples. This is a very close number to 2^31 (i.e. 2 to the power of 31) = 2 147 483648. 2^31 is the biggest positive number that a 32-bit integer can represent, and in the 1.2.x series of Audacity the position in the file is stored as an integer, which will be a 32-bit signed value. So the theoretical maximum number of samples Audacity can store is 2 147 483648, and it follows that the length of time you can record for is then controlled by the chosen sample rate (given you have the disk space to record in the first place of course).

In principle, the solution to recording over 13.5 hours is to use the latest 1.3 Beta version of Audacity. Here, samples are stored as 64-bit values (even on 32-bit machines), which at 44100 Hz sample rate offers a maximum recording length of a mere 58 billion hours. Whilst we believed 64 bit storage worked properly in 1.3.3 and later, some reports in early 2008 suggested that recordings were still coming up against the 2^31 limit. Our development code was fully converted to 64 bit sample counts in July 2008, so long recordings should work again in our Nightly Builds and in 1.3.6 Beta onwards.

If you need/prefer to use Audacity 1.2.x, you can increase recording time over 13.5 hours by reducing the sample rate of the recording. To do this, changing the Project Rate button bottom left of the Audacity screen. For example, recording at 22050 Hz permits a theoretical recording time of just over 27 hours.

Note when exporting your recording that there is a file length limit for WAV and AIFF files. The file size is set in the headers as an unsigned 4 byte integer, which imposes a 4 GB limit, except that some media players interpret the file length as a signed number, which imposes a 2 GB limit.